The present invention relates to digital communication system and more particularly to systems and methods for synchronizing timing between nodes of a digital communication system.
In order to communicate digital data, it is typically necessary to establish one or more forms of timing synchronization between the transmitter and the receiver. For example, in certain systems such as OFDM (orthogonal frequency division multiplexing) systems, data is transmitted in successive bursts and it is necessary to establish a common understanding between a transmitter and a receiver of the time boundaries between bursts. In a TDMA (time division multiple access) system, shared access to a common transmission medium is implemented by dividing the time domain into successive time slots or frames that are allocated to individual transmitters. Nodes of a TDMA digital communication system must establish a shared understanding of the boundaries between frames. In a point to multipoint TDMA communication system where multiple subscriber units exchange data with a common hub or central access point a part of determining the TDMA frame boundaries is establishing round trip propagation delay between the central access point and each subscriber unit in a process known as ranging.
One exemplary ranging protocol is defined by the MCNS protocol developed for cable modem networks. The MCNS protocol is described in the Data-over-Cable Service Interface Specifications, Radio Frequency Interface Specifications, SP-RFI-I04-980724, (Cable Television Laboratories, 1997), the contents of which are herein incorporated by reference.
MCNS allocates contiguous sets of scheduled frames as initial ranging windows in which new subscriber units may transmit a synchronization waveform upstream to the central access point. The central access point measures the time delay between the protocol-defined beginning of the ranging window and the arrival time of the synchronization waveform. This time difference is then communicated to the subscriber unit so that the subscriber unit advances its timing appropriately to have its transmissions arrive within the assigned frame boundaries.
In order to facilitate accurate measurement of the synchronization waveform arrival time at the central access, the synchronization waveform should have several desired characteristics. The synchronization waveform should have a well defined peak in its autocorrelation signal. The synchronization waveform should be time limited so that it fits within the defined ranging window. The spectrum of the synchronization signal should be well confined within a limited frequency allocation. Furthermore, it is also desirable that the synchronization waveform be easily generated.
According to the present invention, a chirp waveform may be employed in establishing timing synchronization between nodes of a data communication network. In one embodiment, the chirp waveform is combined with a waveform modulated with data to form a synchronization waveform to be transmitted. The receiver of the synchronization waveform determines an alignment of the chirp waveform to a template chirp waveform to synchronize timing between nodes.
A first aspect of the present invention provides a method for operating a first node of a digital communication system that includes the first node and a second node. The method synchronizes timing of the second node to timing of the first node. The method includes: transmitting a synchronization waveform including a chirp waveform from the first node to the second node and transmitting data from the first node before the synchronization waveform and after the synchronization waveform.
A second aspect of the present invention operates in a digital communication system including a first node and a second node. The method synchronizes timing of the second node to timing of the first node. The method includes receiving a synchronization waveform from the first node. The synchronization waveform includes a chirp waveform. The method further includes determining an alignment of the chirp waveform to a template chirp waveform to synchronize timing of the second node to timing of the first node.
Further understanding of the nature and advantages of the invention herein may be realized by reference to the remaining portions of the specification and the attached drawings.